“I have to ask myself, ‘Am I in prison, or am I somewhere else?’ It’s surreal,” he tells The Post. “I lived in a small cell for so long. Prison was my life.”

It’s been 16 years since Warner, now 55, was released from the Fishkill Correctional Facility in upstate New York. But that stretch of time is dwarfed by the 21 years he spent locked up for a murder he didn’t commit.

The new film “Crown Heights,” out now, tells the story of Warner (played by Lakeith Stanfield) and his best friend, Carl King (Nnamdi Asomugha). In a shocking case of mistaken identity, institutional racism and tortuous legalese, Warner was wrongly convicted of the murder of another teenager. Even though the key witness changed his story, Warner had an alibi and the real murderer eventually admitted his guilt, the wrongly accused man was kept behind bars for more than two decades, several of those years in solitary confinement. All the while, Warner’s childhood friend, King, fought to prove his innocence.

Warner and King sat down with The Post to look back on those nightmarish years.

The two became friends at the age of 5 in their native Trinidad, before both families moved to Brooklyn. Living in the Crown Heights neighborhood, the Rastafarian teens became familiar with police harassment. “We were teenagers, so we’d gather by the corner,” says King, 54. “They’d come up to us, push us. Say racial slurs. Sometimes we’d get kicked in the behind. Punched. We couldn’t do anything.”

Colin Warner (left) and Carl KingAnnie Wermiel/NY Post

Then on April 10, 1980, a 16-year-old named Mario Hamilton was killed with a gunshot to the head on Flatbush Avenue. Two 14-year-olds were interrogated by the cops: Thomas Charlemagne, the only witness to the murder, and Hamilton’s brother, Martell. Exhausted after hours of interrogation, Charlemagne picked Warner’s mugshot out of the police book. Warner had one prior, nonviolent offense for carrying a switchblade, a charge for which he was still serving three years’ probation.

Martell, when questioned with Warner’s photo, “put me on the scene, not knowing me,” says Warner. “They said, ‘Have you seen Colin Warner around?’ And he said, ‘Yes.’ But you have to understand where he was coming from, as the victim’s brother. He was grieving.”

Warner was arrested based on those two juveniles’ statements, despite the lack of evidence tying him to the crime. Warner did not know Hamilton at all.

“It happened out of the blue,” says Warner, who was 18 at the time. “The police come to my house, for me, in connection with a murder. It was totally off the grid for me. Even though I knew I wasn’t a goody-goody, my level of aggression didn’t reach to the level of taking another human life.”

King, 17 at the time, had not been with Warner when the murder took place, but he knew who had.

“Two of my friends were with Colin at the time. They gave that information to the police. They gave statements as to Colin’s whereabouts. They knew he was innocent,” King said.

A second man, 15-year-old Norman Simmonds, was arrested months later for being the driver of the car that Warner was supposed to have taken before and after killing Hamilton. The two men, who had never met, were tried together two years after Warner was arrested.

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During the trial, Simmonds confessed to Warner that he had really committed the crime. “I came into contact with a mentality that I didn’t know existed,” Warner says. “He told me he did it.”

But Simmonds refused to tell anyone else that Warner was innocent, and he rejected a plea deal that would have exonerated Warner.

“The only hope I had in trial was when the court officers placed bets on whether I was going to win or lose,” Warner says. “They had more money on me winning the trial than losing. My case was like a circus, literally.”

The first trial ended in a hung jury, with Charlemagne changing his testimony to say that only Simmonds had been the killer in a drive-by shooting of Hamilton. But the second trial still resulted in a charge of second-degree murder for both, partially due to Charlemagne’s initial testimony.

As a juvenile, Simmonds got nine years to life; Warner got 15 to life.

“When Colin was convicted, he had an immediate appeal put in,” King says. “But it took six years. And in between that time, we are thinking that the appellate court will see something wrong that the trial court didn’t. But they didn’t.” The appeal was denied.

“Part of the decision from the appellate court read, ‘The evidence against Mr. Warner is far from overwhelming,’ ” Warner said. “Imagine you’re in court, on trial, and the court is telling you the evidence they used to convict you was far from overwhelming. And yet you still have a life sentence. This is the kind of joke my life came to be.”

With no money for a private lawyer, Warner had been at the mercy of overworked public defenders, who often failed to show up when they said they would and left him in limbo waiting for trial appearances. King — who faithfully made hours-long trips to visit his friend in various prisons — would raise money to hire a different lawyer, only to realize the new hire knew less than he did about the case.

In his early years in prison, Warner got into fights, unable to contain his fury and frustration toward the system’s indifference. “I don’t give a f–k about nothing because I’m innocent! You can’t treat me like a guilty person!” Warner remembers himself thinking. He would spend four years in solitary confinement.

Lakeith Stanfield as Colin Warner in the film “Crown Heights”

Gradually, he learned to channel his emotions differently, going back to school in the prison system. “I could not have kept that anger flowing like a river — it would have destroyed me in prison,” he says. Eventually, he earned an associate’s degree in liberal arts.

“Every step of every day, there was always the hope that somebody would stop this s–t,” Warner says. “I don’t know how to explain it. You feel you’re under pressure from bullies. From the courts, the detectives, the jails. Everybody’s all over you. You’re being bombarded.”

King took a job as a court process server at the age of 33, work that allowed him entry to the legal system. He began to teach himself how to be an investigator.

King paid dearly for his dedication to overturning Warner’s conviction: He lost his savings, and his marriage fell apart. But King has no regrets.

“We grew up in a village — you know, ‘It takes a village.’ So it was natural to me . . . as far as trying to do something to make a difference.”

Finally, in 2001, King found a young lawyer, William J. Robedee, who believed in Warner’s innocence and who joined King in tracking down Simmonds, who had been paroled in 1989, on Long Island.

A news clip from the New York Post of Colin Warner’s release from Fishkill Correctional Facility

Simmonds had previously signed an affidavit in 1991 stating that he had acted alone in the murder, but it was deemed insufficient by the court.

King and Robedee, who had just started his private practice after a short stint in the DA’s sex crimes unit, persuaded Simmonds to give them a full deposition admitting he had murdered Hamilton alone. After 21 years in prison, King says, it only took 21 days for them to get in front of a judge and free his friend.

King, who still lives in Crown Heights and has continued to work as a process server, recently started his own nonprofit, Success to Freedom, dedicated to helping others who have been wrongly convicted.

Warner — who was awarded $2.7 million in 2009 for his wrongful conviction — moved to Georgia with his wife, Antoinette, and their daughter. “I own a couple of properties in New York and Georgia as rentals,” he says, “and I also own a shelter which houses about 20 guys, that the state of New York pays me for. It’s a [prison] halfway house, but it’s transitional. We try not to let them stay there too long. Little steps. I never thought I would get involved in something like that. But it’s something to do, and it’s helping people.”